The practice of sharing syringes without adequate sterilization between successive users is a major contributor to the transfer of Human Immunodeficiency Virus (HIV) and Hepatitis with subsequent severe repercussions for the sufferer and at a high cost to society for supporting and providing medical attention to sufferers. Further problems arise for health professionals administering medicines and vaccines to infected individuals, where accidental needle stick injury by a used syringe can lead to infection.
In response to this problem, syringes have been developed which either provide a needle sheathing mechanism or a needle retraction mechanism to prevent re-use and/or needle stick injury. However, many such retractable syringes have highly specialized retractable needle assemblies that are not amenable to replacing needles which have been bent or burred or for allowing a user to select an appropriate needle size. An additional complication is that it is difficult to ensure that retractable needles do not inadvertently move proximally (i.e., retract) into the barrel such as when piercing the skin prior to injection, while not resisting retraction after completion of injection. Retaining systems that achieve this balancing act can be difficult to design, particularly where the sometimes competing interests of costs of manufacture and reliability have to both be satisfied in a commercially-viable retractable syringe. These problems can be even more marked in the context of a retractable syringe having a replaceable, retractable needle assembly.